1. Technical Field
The present invention relates to a computer power supply unit, and in particular to a power supply unit which operates efficiently under a wide range of load conditions from light to heavy within a computer.
2. Description of the Related Art
In a personal computer (PC) a DC (direct current)/DC converter is typically provided for converting an applied DC voltage into various DC voltage levels, such as 2.5 V or 3.3 V, that are supplied to different parts (load) of the PC. Among the types of power supply circuits that can be used as such DC/DC converters are switching power supply circuits and series power supply circuits.
FIG. 8(A) illustrates an example power conversion efficiency measurement for a switching power supply circuit (commercially available as an integrated circuit (IC) chip) wherein the input voltage is 3.3 V and the output voltage is 2.5 V. As depicted in FIG. 8(A), the switching power supply circuit has a low power conversion efficiency when the output current (load current) is small, i.e., under a light, and improves as the output current increases, i.e., a heavier load. In the example shown in FIG. 8(A), the power conversion efficiency decreases dramatically when the output current is less than 20 mA and improves to 75% to 83% when the output current exceeds 20 mA.
With reference to FIG. 8(B), there is depicted an example power conversion efficiency measurement that is characteristic of a series power supply circuit (commercially available as an IC chip) wherein the input voltage is 3.3 V and the output voltage is 2.5 V. As illustrated in FIG. 8B, the power conversion efficiency is not substantially altered in accordance with load changes. While the ideal power conversion efficiency of a series power supply circuit when the input voltage is 3.3 V and the output voltage is 2.5 V is about 76% (=2.5/3.3′100), actual power conversion efficiencies are within the range of 65% to 72% as depicted in FIG. 8(B) due to environmental influences such as a circuit loss.
As depicted with reference to FIGS. 8(A) and 8(B), the power conversion efficiency of both types of power supplies suffers at either end of the load spectrum, i.e., low power conversion efficiency for switching power supply circuits under light loads and lower power conversion efficiency for series power supply circuits under heavy loads relative to switching power supply circuit.
A power supply unit that provides high power conversion efficiency under a wide range of loads from light to heavy is required for portable PCs such as notebook PCs, sub-notebook PCs, palmtop PCs, and personal data assistants (PDA), which alternate between active mode and suspend mode (a low load condition).
Technologies that address the above-mentioned problems are described in Published Unexamined Patent Application Nos. 11-8933 and 8-149804. The technology described in Published Unexamined Patent Application No. 11-8933 provides a first stabilized power supply circuit (a series power supply circuit) supporting lighter loads and a second stabilized power supply circuit (a switching power supply circuit) and activates or inactivates the second stabilized power supply circuit depending on the amount of load current to accommodate load variations. The technology described in Published Unexamined Patent Application No. 8-149804 provides a first switching device for smaller currents and a second switching device for larger currents in a switching power supply circuit and switches between the first and second switching devices corresponding to the load current value, thereby reducing the switching device power and improving the power conversion efficiency under a load current variation condition.
A higher precision output voltage must be provided for both of the series and switching power supply circuits described in the above-mentioned Published Unexamined Patent Application No. 11-8933 since the cumulative output voltage is provided by both of the power supply circuits. Accordingly, higher precision components must be used, or the number of components must be increased, resulting in higher costs.
For example, letting VTYP be the average output voltage and 5% be the precision required by load electronics (such as ICs, LSIs), the maximum output voltage Vmax is VTYP+5% and the minimum output voltage Vmin is VTYP−5%. Assuming that the output voltage, V10, of a series power supply circuit is controlled so as to be within a range from VTYP+1% to VTYP+5%, and the output voltage, V30, of a compound power supply circuit consisting of the series power supply circuit and a switching power supply circuit is controlled so as to be within VTYP−5% to VTYP+1%, an output precision of ±2% is required for the series power supply circuit, and an output precision of ±3% is required for the switching circuit. To achieve such precision, high-precision components or an increased number of components are required compared to a design for providing an output precision of ±5%.
The system described in the above-mentioned Published Unexamined Patent Application No. 11-8933 has a lower output voltage precision due to a voltage drop over an output load current detection resistance on the output side. The system described in the above-mentioned Published Unexamined Patent Application No. 8-149804 also suffers a reduced lower output voltage precision due to a voltage drop over an output load current detection resistance on the output side.
It would therefore be useful to provide a power supply unit that provides a higher efficiency and output voltage precision under a wide range of loads.